1. Field of the Invention
The present invention generally relates to a photolithography process. More particularly, the present invention relates to a method of reducing a critical dimension (“CD”) bias between a dense pattern and an isolation pattern on a chip.
2. Description of the Related Art
In the design of a semiconductor device, the integration of a circuit device grows up rapidly and the dimension of the whole circuit device reduces rapidly. The most important process of confining the dimension of the whole semiconductor process is the photolithography process. For example, in a device structure related to a metal oxide semiconductor (“MOS”), such as a pattern of film layer or an area having dopants, the dimension is confined by the photolithography process. In addition, whether the line width of the device, and so integration of the whole semiconductor process, may or may not be less than 0.18 μm, is dependent on the development of the photolithography process.
In general, the area of a mask may be separated into at least a portion of isolated pattern, and another portion of dense pattern. For example, in a system on a chip (“SOC”) process of manufacturing a logic circuit and a memory cell, such as a read only memory, a static random access memory, a flash memory or a dynamic random access memory on the same chip, the logic circuit relates to an isolation pattern and the memory cell relates to a dense pattern in general. However, when an exposure process is used for transferring the isolation pattern and the dense pattern to a photoresist layer, a CD bias between a dense pattern and an isolation pattern is produced by a flare effect, and the efficiency of the manufactured device is reduced.
A conventional method of resolving the flare effect is an optical proximity correction (“OPC”) method, which method relates to decrease or increase of the line width of the original pattern, in order to have the same line width in the transferred isolation pattern and the transferred dense pattern. However, because of the flare effects produced by different exposure apparatus are different from each other, each original pattern may have different correction parameters on different exposure apparatus. Moreover, the process windows between an isolated pattern and a dense pattern for a corrected pattern are different, and the flare effect is also dependent on the pattern density. Thus the OPC method can not resolve the CD bias and the difference of the process window produced by the flare effect.